This invention relates in general to crankshafts for internal combustion engines and, more specifically, to a system using two geared together, contra-rotating, crankshafts, connected to a piston through two crossed connecting rods each driven by one of the crankshafts.
In conventional internal combustion engines, each piston drives a single crankshaft through a single connecting rod extending between a wrist pin centrally located in the piston and a crankshaft pin. This arrangement is simple, light weight and has been brought to a high degree of development. However, this arrangement has problems with balance, noise and sidewall thrust on the piston resulting in undesirable friction. Consumers continue to demand smoother, more efficient, quieter engines. Automobile manufacturers have implemented engine balancing aids, primarily in the form of rotating balance shafts. Balance shafts are parasitic devices that improve balance but create durability problems, increased cost, complexity and weight as will as reduced engine efficiency. Off-center piston forces, noise and side thrust problems remain.
A number of different engines have been designed using two crankshafts with two spaced connecting rods connected to a single piston wrist pin to improve engine balance. Typical of these are the arrangement described by Powell in U.S. Pat. No. 1,433,649, Holman in U.S. Pat. No. 2,392,921 and Deland in U.S. Pat. No. 4,690,113. Very complex linkages are required to allow connection of two connecting rods to a single wrist pin and achieve the required linear piston movement. Torque between the two connecting rods may not be uniform and over-all engine balance is little improved.
Others have provided two crankshafts connected by two spaced connecting rods to two spaced wrist pins in attempts to provide more linear, balanced, piston movement. Typical of these are the arrangements described by Porter et al. in U.S. Pat. No. 810,347, Milano in Italian Patent No. 445,002 and Taga in Japanese Published Application No. 55-159947. Improved engine balance and reduced sidewall thrust are said to be achieved by this system. However, very close machining tolerances are required and these designs are sensitive to tolerance "stack up".
Mandella, in U.S. Pat. No. 5,211,065, describes an arrangement in which a complex assembly of connecting rods and linkages is provided between a single crankshaft and two spaced piston wristpins with the connecting rods crossed in order to achieve simple harmonic motion of the piston. This may improve cylinder axis balance but does nothing to overcome side-to-side balance and noise.
Thus, there is a continuing need for improvements in the relationship of crankshaft, connecting rods and pistons in internal combustion engines in order to improve balance, provide better torque, reduce sidewall thrust, reduce piston to cylinder friction and engine noise while reducing sensitivity to machining tolerances and tolerance stack-up.